This invention relates to a technique for converting oxygenated aliphatic compounds, such as methanol or dimethyl ether, to lower olefins. In particular, it provides an efficient continuous process for producing an olefinic product rich in C.sub.3.sup.+ alkenes, while minimizing byproduct aromatics and ethene. In view of the availability and low cost of synthetic methanol (MeOH) and its corresponding dehydration product dimethylether (DME), primary emphasis will be placed on these oxygenates in the following description of the methanol-to-olefin (MTO) process.
Various zeolitic catalysts are useful for converting methanol and other lower aliphatic alcohols or corresponding ethers to olefins. Recent interest has been directed to a catalytic process for converting methanol over ZSM-5 catalysts and related to valuable hydrocarbons rich in ethene and C.sub.3.sup.+ alkenes. Various processes are described in U.S. Pat. Nos. 3,894,107 (Butter et al); 3,928,483; 4,025,575; 4,252,479 (Chang et al); 4,025,572 (Lago); and in copending U.S. patent application Ser. No. 388,768, filed June 15, 1982 (Yurchak et al), incorporated herein by reference. Significance of the methanol-to-olefins (MTO) type processes, especially for producing ethene, is discussed in Hydrocarbon Processing, November 1982, pp. 117-120. It is generally known that MTO processes can be optimized to produce a major fraction of C.sub.2 -C.sub.4 olefins; however, a significant C.sub.5.sup.+ byproduct may be coproduced, including polymethylbenzenes such as durene, as described in U.S. Pat. No. 4,025,576 (Chang et al). Prior process proposals have included a separation section to recover ethene and other gases from byproduct water and C.sub.5.sup.+ hydrocarbon liquids.
Fluidized bed MTO processes provide an important economic unit in the synthesis route for producing various chemicals, fuels and other hydrocarbon products from coal and/or natural gas. As disclosed in copending U.S. patent application Ser. No. 548,377 filed Nov. 3, 1983 (Tabak) and Ser. No. 598,955 filed Apr. 11, 1984 (Hsia et al), incorporated herein by reference; methanol, DME or the like may be converted to liquid fuels, particularly distillate, in a multi-stage continuous process, with integration between the major process units. The initial stage MTO process hydrocarbon olefins effluent can be fed to a catalytic oligomerization unit. Although the C.sub.3.sup.+ mono-olefins are readily converted by ZSM-5 type acid zeolites to produce olefinic oligomers, ethene tends to be less reactive at moderate temperature, and may be removed from the oligomerization system. Thus, minimizing ethene production and increasing C.sub.3.sup.+ olefin in the MTO product stream is advantageous, and this is a main object of the present invention.